Synthetic fibers are mainly used in clothing applications, and many considerations have come to be actively made for polymer modification, modifying cross sections, imparting functionality, increasing fineness, and the like in order to improve the performance and texture thereof. In particular, the increased fineness of single fibers has led to the progression of suede-tone artificial leather from the development of micro fibers, and this basic technology thereof is employed in life materials such as wiping cloths and industrial material applications like filters, and thus currently, further increases in fineness are continuing. Nowadays in particular, the use of nanofiber nonwoven fabric is being actively considered in secondary battery separators equipped to hybrid vehicles and electric cars, filters with improved high functionality, etc.
The size of the fine holes in a fibrous bundle such as non-woven fabric is said to be greatly influenced by the diameter of the single fibers constituting the fibrous bundle. In other words, in order to make smaller fine holes to form, it is necessary to form a non-woven fabric with smaller fibers in fiber diameter. However, with conventional spinning methods based on melt spinning, wet spinning, etc., about 2 μm is the limit to thinning the fiber diameter, and it has not been at a level that adequately responds to the needs for nanofibers.
As one of the production technologies of nanofibers, the phase-separation method has been known industrially. This is a technology that sea-island conjugates or blend spins two types of polymer components that are in separate phases from each other, removes the sea component from the solvent, and makes the remaining island component into nanofiber. For the nanofibers of this system, drawing can be conducted in the same way as typical fiber structures; therefore, the degree of orientation of molecules and degree of crystallization are high, and fibers of relatively high strength are obtained.
However, after spinning or after non-woven fiber manufacture, an abundant amount of the sea component must be removed from the solvent, which has become a cause of a cost increase due to the recovery or waste treatment of the removed sea component being necessary. At the same time, these treatments have not been preferable in terms of the environment either. In addition, the single-fiber fineness of the nanofibers obtained herein is determined by the dispersion state of the island polymer in the sea-island polymer fiber; therefore, concern has remained over the uniformity of the fiber diameter such as variation of the single-fiber fineness of the obtained nanofibers becoming great, if the dispersion is insufficient.
As one other method for production technology of nanofibers, there is the electrospinning method. This method produces fine nanofibers by electrostatic repellent force, by way of applying high voltage between the spray nozzle and the counter electrode upon ejecting a macromolecule solution or the like from a spray nozzle, thereby causing an electric charge to accumulate on a dielectric inside of the spray nozzle. When ejecting nanofibers from the spray nozzle, the polymer is made finer by the electrostatic repellant forces, and thus a nanoscale fine fiber is formed. At this time, the solvent causing the polymer to dissolve is released out of the fiber, and almost no solvent is contained in the deposited nanofiber. Since the nanofiber bundle of an almost dry state is formed immediately after spinning, it is considered a simple production process.
However, the electrospinning method remains with a big problem in the productivity of industrial scale. In other words, since the production volume of nanofibers is proportional to the number of spray nozzles, there is a limit in the technical issue of how much the number of spray nozzles is increased per unit area (or space). In addition, since the polymer ejection volume from each spray nozzle is not fixed, there is a problem in variation in fiber diameter and variation in deposited amount in the non-woven fabric, problem of strength being weak due to drawing not being possible, problem in not being usable by making into short fibers, etc.
In addition, the occurrence of corona discharge can be given as a problematic issue in production derived from using spray nozzles. When a corona discharge occurs, the applying of high voltage to the spray nozzle tip becomes difficult, and the accumulation of sufficient electric charge to the polymer solution inside the spray nozzle is not carried out, and thus it becomes difficult to form nanofibers. Although various methods for suppressing this corona discharge have been considered, the solution has been difficult.
The problem in the productivity from employing such an electrospinning method is derived from using spray nozzles; therefore, considerations of electrospinning methods that do not use spray nozzles are also being carried out. For example, there is a method using a magnetic fluid as an electrode, and performing electrospinning from a macromolecular solution surface, and due to not using spray nozzles, spinning with easy maintenance can be realized, and it has been possible to rapidly improve the spinning rate. However, there remains the problem of the spinning state being very unstable with this method.
As another spinning method that does not use spray nozzles, an electrospinning method using a rotating roll has been proposed. This method is a method of immersing the rotating roll in a bath filled with the polymer solution, thereby attaching the polymer solution onto the roll surface, then applying high voltage to this surface, and performing electrospinning. When compared with a conventional electrospinning method, this has been a ground-breaking method in aspects of the productivity improvement and ease of maintenance. However, there is a limit in the area of the rotating roll portion to be spun, and thus there has been a problem in being necessary to increase the rotating roll diameter or increase the number of rotating rolls in order to further raise productivity, which leads to a size increase in the production facilities.
In addition, a production method of nanofiber masses has been proposed that causes a polymer fiber jet to fly from the polymer solution surface and pile up, by incorporating an apparatus to cause air bubbles to form in the bath of polymer solution to which high voltage is applied. However, with this method, upon causing foam to form at the surface of the polymer solution and causing the polymer fiber jet to fly from the top of the foam, there is a problem in that the fine spray from the breaking off of the foam will fly and adhere to the nanofiber surface.
With the electrospinning method, further to there being a limit in the productivity and stability of the product, a new large investment is required; therefore, the present inventors have considered there to be a possibility to establish technology that effectively applies a conventional wet-spinning facility to produce continuous nanofibers with little fiber diameter unevenness by way of a direct spinning method, while suppressing new investment expenditures.
As production methods of fibrous bundle (continuous long fiber bundles) consisting of ultrafine fibers by way of a wet-spinning method, various technologies related thereto are disclosed in the publications given next.
Patent Document 1 (Japanese Unexamined Patent Application, Publication No. 2000-328347) describes a spinneret and a production method of acrylic fibers, and describes raising the hole density to 3 to 35 holes/mm2, and being used to wet spin acrylic fibers with a single-fiber fineness of 0.03 to 50 denier.
Patent Document 2 (Japanese Unexamined Patent Application, Publication No. S62-21810) describes a square-shaped nozzle for wet spinning, and describes being able to stably spin 1.5 denier fiber without breaking from a spinning nozzle defining the width, length and block inter-distance of the spinning hole blocks are specific distances, and having a hole density of 16.6 holes/mm2.
Patent Document 3 (Japanese Unexamined Patent Application, Publication No. S51-119826) describes a ultrafine fibrous bundle, production method thereof and a production apparatus thereof, and describes using a spinneret made from a sheet sintered plate made from metallic fiber having a filtration accuracy of at least 15 μm to obtain a ultrafine fibrous bundle having non-uniform fiber cross-section with severe unevenness at 0.01 to 0.5 denier, by way of wet spinning.
The ultrafine fibrous bundle obtained in this way has come to be widely used as life materials including clothing and industrial materials, as already mentioned; however, particularly in recent years, nanofiber non-woven fabric (synthetic paper) made using ultrafine fibers have come to be abundantly used as secondary battery separators equipped to hybrid vehicles and electric cars, filters with improved high functionality, etc. as described and proposed in Patent Document 5 (Japanese Unexamined Patent Application, Publication No. 2012-72519), for example. Conventionally as well, synthetic paper for which synthetic fibers are the raw material have come to be utilized in battery separators, oil filters, electronic wiring substrates, etc. due to having little variation in dimensions from water absorption compared to paper with cellulose as the raw material.
In the past, synthetic paper with synthetic fiber as the raw material came to be utilized in battery separators, oil filters, electronic wiring substrates, etc. due to having little variation in dimensions from water absorption compared to paper with cellulose as the raw material.
On the other hand, as described in Patent Document 4 (Japanese Unexamined Patent Application, Publication No. S58-7760), for example, acrylic fiber paper produced by papermaking the acrylic fibers produced by wet spinning is one of the materials that has come to be widely used in the field of synthetic paper from long ago. Contrary to polyester fibers and polyolefin fibers, since acrylic fibers do not melt fuse even when performing hot calendar processing due to hardly exhibiting thermoplasticity, as well as being hydrophilic and thus excelling in chemical resistance, the acrylic fiber paper has come to be widely used in fields such as the separators of alkali batteries.
The above-mentioned Patent Document 5 describes that, if consisting of an acrylonitrile copolymer obtained by blending at least 93% by mass of acrylonitrile, and the single-fiber fineness is no more than 1.0 dtex, it is preferable because the intertwining of fibers will be moderate upon papermaking, and describes that, if in the range of at least 0.01 dtex to no more than 0.2 dtex, it is more preferable because the uniformity in the papermaking process will be superior, and the industrial productivity can also be ensured.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2000-328347
Patent Document 2: Japanese Unexamined Patent Application, Publication No. S62-21810
Patent Document 3: Japanese Unexamined Patent Application, Publication No. S51-119826
Patent Document 4: Japanese Unexamined Patent Application, Publication No. S58-7760
Patent Document 5: Japanese Unexamined Patent Application, Publication No. 2012-72519